U.S. Pat. No. 7,204,868 describes a conventional aircraft fuel system with a fuel tank, a venting system for reducing pressure difference between the interior and exterior of the fuel tank, and an inerting system for supplying oxygen-depleted gas to the fuel tank. During descent, oxygen-rich air flows into the tank via the venting system and nitrogen-enriched air (NEA) from an air separation module (ASM) is injected into the venting system to pre-inert the vent in-flux. During a fast descent, NEA air is also injected into the tank via a separate set of inerting nozzles.
This system is good during ideal descent cases but for descents that include, for example, loiter or go-around, i.e. where the aircraft levels out in altitude or ascends, the NEA within the surge tank and vent line will expel overboard, which does not reduce the burden on ASM performance and so the ASM size (and therefore weight and cost) is not optimised.
Another problem with the arrangement of U.S. Pat. No. 7,204,868 is that a large number of heavy vent pipes and vent nozzles are required in order fully vent all parts of the fuel tank.